Scientists from Samara University and the RAS Lebedev P.N. Physical Institute have developed and constructed an experimental levitation setup designed for testing and selecting components of highly efficient, low-toxicity next-generation rocket propellants. The apparatus enables experiments with droplets of various substances under acoustic levitation conditions—where test liquid droplets float suspended in air, held in place by an ultrasonic wave field.
Levitation conditions offer greater precision and visual clarity in chemical experiments compared to traditional approaches: suspended droplets do not contact the walls of containers, minimizing external interference with chemical reactions. Moreover, researchers can remotely manipulate the floating droplets without physical contact. Experiments are already underway on the levitator using hypergolic ionic liquids*, which may become components of promising next-generation rocket fuels. Droplets of ionic liquids with varying compositions, guided by invisible ultrasound, merge with oxidizer droplets while scientists observe the resulting ignition and combustion reactions, fine-tuning the optimal "formulation" for future space propellants.
The project received financial support through a grant from the Russian Science Foundation. Researchers from the Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences are also participating in the studies.
"Hypergolic ionic liquids are attracting significant attention from scientists worldwide. These energetic liquids can serve as the basis for new types of space propellants that are more efficient and environmentally friendly. We proposed using a novel device—an acoustic levitation setup, or simply a levitator—for the experimental study of ionic liquids. Our levitator has already been assembled, tested, and is now being used for a series of experiments with promising hypergolic ionic liquids," said Ivan Antonov, Associate Professor at the Department of Optics and Spectroscopy of Samara University.
According to the researcher, acoustic levitation offers experimenters several critically important advantages over conventional methods for studying ionic liquids. The setup securely holds individual droplets of chemically distinct substances at different nodes of an acoustic standing wave and allows remote manipulation: droplets can be moved through space and merged with one another using low-frequency modulation of the ultrasonic carrier wave. As a result, chemical reactions occur literally in open space, free from the influence of test tube or combustion chamber walls.
"The absence of walls contacting the droplet—so-called 'containerless conditions'—eliminates the effects of undesirable surface reactions and heat exchange processes typical in bulk liquid volumes. Furthermore, the levitator enables combustion studies under microgravity-like conditions—almost as in space—without requiring complex specialized test stands. The levitator provides a unique opportunity to study reactions at the level of individual droplets and to investigate how the size of a single droplet—from several hundred micrometers to a couple of millimeters—affects its chemical activity. Ultrasonic levitation can also support research in other areas, such as deeper investigation of fuel combustion and injection processes in engines, which could lead to the development of more reliable and highly efficient propulsion systems in the future," noted Ivan Antonov.
Externally, the levitator resembles a multi-tiered rack with dozens of ultrasonic emitters mounted on upper and lower platforms, facing each other like miniature rocket nozzles or oven burners. According to Ivan Antonov, the uniqueness of the system lies in its special emitter configuration and proprietary control algorithms that enable selective manipulation of individual droplet positions. The equipment suite integrated into the setup not only allows sophisticated control of floating droplets but also supports emission spectroscopy and high-speed optical diagnostics.
Ongoing research on the setup aims to identify correlations between the molecular structure of ionic liquids and their hypergolicity—that is, their ability to spontaneously ignite upon contact with an oxidizer. Based on discovered patterns, scientists hope to obtain new hypergolic ionic liquids with optimized properties and develop novel synthesis methods.
As part of the initial experimental series, researchers conducted the first comprehensive study of the ignition process for several promising hypergolic ionic liquids upon contact with red fuming nitric acid. Using high-speed video recording, ignition delay times were measured; for some liquids, these values were two to three times lower than those obtained in standard tests—indicating the levitator's potential for more accurately modeling real-world conditions.
In the near future, a series of experiments is planned on the levitator with a new class of ionic liquids developed at the Zelinsky Institute of Organic Chemistry. These compounds contain various energetic anions, feature high nitrogen and oxygen content, and exhibit detonation characteristics comparable to trinitrotoluene (TNT), while demonstrating low sensitivity to impact.
"In the presence of catalytic additives, these ionic liquids display hypergolic properties—igniting spontaneously upon contact with oxidizers such as hydrogen peroxide or nitric acid. This opens promising prospects for their use as safe and efficient components in fuel systems," emphasized Ivan Antonov.
For reference:
An ionic liquid is a substance composed entirely of ions. Broadly defined, it includes any molten salt. Ionic liquids belong to the category of so-called "green solvents" and find applications in biotechnology, energy, chemistry, and rocketry. Hypergolic ionic liquids are ionic liquids that spontaneously ignite upon contact with an oxidizer.
Photo by Olesya Orina
